There are known measuring systems as, for example, systems in numerical control machine tools for detecting the position and/or the dimensions of workpieces, by checking heads, or contact detecting probes mounted in the machine, that in the course of the checking cycles displace with respect to the workpiece, touch the surface to be checked and, after contact has occurred, cause the wireless transmission of signals to receiving units.
Each receiving unit is in turn connected, by means of an interface unit, to a relevant numerical control unit that, by processing other signals indicative of the position of the probe, receives information about the position of the workpiece surfaces.
The probes can include electric batteries for the power supply of the circuits detecting contact and the transmission devices. The wireless transmission can occur, for example, by emitting electromagnetic signals of optical or radio-frequency type.
Since a probe is utilized just for short time intervals during the machining cycle of its machine tool, it is normally kept in a "stand-by" condition of low power consumption and it is powered up only when there is the need to perform a checking cycle.
The switching from the "stand-by" condition to the "powered-up" state can be carried out by controlling suitable switching devices on the probe by means of wireless activation signals sent by the receiving unit.
A system of this type is illustrated and described in U.S. Pat. No. 4,693,110.
In a working environment, for example in a workshop, there can be multiple checking probes, installed in various machine tools: each machine tool can comprise, in general, one or more probes for performing checking cycles and sending associated signals to the receiving unit of that machine. Generally, for each machine tool just a probe at a time is selected to perform a checking cycle, even though there are cases in which, in specific machines, two (or, in theory, more) probes perform checkings in partially coincident moments.
When an activation signal is sent by the interface of a machine for energizing a specific probe in the machine that has to perform a checking cycle, it is advisable to avoid that other probes in that machine or in adjacent machines be concurrently activated.
The need to activate an individual probe and not others operating in the same environment is particularly evident when transmission between the probe and the receiving unit occurs (in both ways) by means of radio-frequency modulated signals, as in this case, unlike to what occurs, for example, in optical-type transmissions, it is practically impossible to assign a precise direction to the emitted radiation, and a single activation signal "awakes" all the probes in a specific working area.
A radio telemetry system in which a master station activates a plurality of transponders is disclosed by EP-A-0428322. In the system therein described, the transponders are meters and the master station, that is movable, has to collect the data reported from all of them, one by one, and does so, awaking them from a stand-by state, in a substantially random order. In fact, the order depends on different circumstances occurring at the moment the awaking action starts (e.g.: which of a number of radio channels is the quietest; which of the meters is listening on that channel; which of the awoke meters is the nearest to the movable station; . . . ).
As a consequence, the system shown in EP-A-0428322 cannot be used in a measuring system in which a selected, specific probe has to be activated.